1. Field of the Invention
This invention relates to a photosensitive element having a layer of powder material, and in particular to a photopolymeric element useful in flexographic printing having an overall layer of powder on a surface of the photopolymerizable layer. The invention also relates to methods for making a flexographic printing plate from the photosensitive element.
2. Description of Related Art
Flexographic printing plates are well known for use in printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, plastic films, etc. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element, and a release layer on the photopolymerizable layer to prevent adherence of a phototool. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable solvent removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
Imagewise exposure of a photosensitive element requires the use of a phototool which is a mask having clear and opaque areas covering the photopolymerizable layer. The phototool prevents exposure and polymerization in the opaque areas. The phototool allows exposure to radiation in the clear areas so that these areas polymerize and remain on the support after the development step. The phototool is usually a photographic negative of the desired printing image.
In some applications, it is desirable to eliminate the phototool by directly recording information on a photosensitive element, e.g., by means of a laser beam. In particular, digitized imaging without a phototool is well-suited for making seamless, continuous printing forms. The image to be developed could be translated into digital information and the digital information used to place the laser for imaging. The digital information can be transmitted from a distant location, corrected easily and quickly by adjusting the digitized image, and precisely controlled by a machine during imaging. Digitized imaging can also reduce storage space and costs by eliminating the need for positive- and negative-working photosensitive materials or positive and negative phototools.
In general, it has not been very practical to use lasers to image the photopolymerizable layer of the elements which are used to prepare flexographic printing plates. The elements have low photosensitivity and require long exposure times even with high powered lasers. In addition, most of the photopolymerizable materials used in these elements have their greatest sensitivity in the ultraviolet range. While UV lasers are known, economical and reliable UV lasers with high power, such as the ion laser, are generally not available. In UV lasers, such as the excimer laser, the laser cannot be modulated quickly enough to create a precise image at high write speeds. However, non-UV lasers are available which are relatively inexpensive, and which have a useful power output and which can be utilized to form a mask image on top of flexographic printing elements.
U.S. Pat. No. 5,262,275 and pending U.S. patent applications Ser. No. 08/130,610 and Ser. No. 08/341,731, describe a photosensitive element and processes for making flexographic plates which use laser radiation to form an image on a layer on the element. The element comprises a support, a photopolymerizable layer, at least one barrier layer, and at least one layer of infrared radiation sensitive material. The process includes the step of imagewise ablation of the layer of infrared radiation sensitive material with infrared laser radiation to form an in-situ mask. The element is then exposed to actinic radiation through the mask and treated with developer solution. The infrared-sensitive layer includes an infrared-absorbing material, a radiation-opaque material (which can be the same as the infrared-absorbing material) and optionally, a binder. The infrared-sensitive layer can be prepared by coating the infrared-sensitive material onto a temporary coversheet or directly onto the barrier layer on the photopolymerizable layer. If the infrared-sensitive layer is on the temporary coversheet, the infrared sensitive layer is placed on the photopolymerizable layer (barrier layer) of a second element and the layers are pressed together.
U.S. Pat. No. 5,262,275 and the related applications disclose coating (i.e. materials in solution) as a method to form the infrared sensitive layer. The infrared sensitive layer can be formed by coating the infrared radiation sensitive material in one pass or in multiple passes. However, the various methods of coating do not always provide the necessary high covering power for the infrared sensitive layer. Covering power relates to the ability of a composition to form a layer relative to density (optical) and coating thickness, i.e., coating weight, of the layer. High covering power of a composition, e.g., infrared radiation sensitive material, achieves high optical density with a minimum coating thickness of the layer. Generally, a composition having poor covering power provides a thick coating layer in order to reach desired optical density. Poor covering power results in lower ablation sensitivity, detrimentally affecting ablation performance and thus the ultimate print productivity of the plate. Further, when the infrared sensitive layer is coated from an infrared sensitive material in solvent solution, the infrared sensitive layer may develop cracking or crazing appearance on the photosensitive element. This can particularly occur when the infrared sensitive layer is coated onto a cylindrically-shaped photopolymeric layer directly.
The process of desolvation-removing solvent from the coating layer, may result in plates or cylindrical plates which develop cracks or weaknesses in the coating layer over time. There is a need, therefore, for a flexographic printing element which does not have such problems. The present invention meets this need by providing flexographic printing elements and plates wherein an infrared ablatable composition which forms a mask or a release composition is manufactured or produced which does not have a discrete film-like infrared ablatable layer or release layer. This invention also provides seamless coverage for the cylindrical flexographic printing plate having an IR ablatable layer.
U.S. Pat. No. 4,229,518 describes a photohardenable element useful for color proofing which contains a protective topcoat. The protective topcoat comprises a layer of a tacky material dusted with a powdered material capable of rendering the layer non-tacky and the element suitable for further handling. The tacky material includes unexposed photohardenable monomer and plasticizer. The application of the topcoat is a final step in the formation of a toned image-bearing element. Thus the image-bearing element having a layer of powdered material as a topcoat is not subjected to any additional process treatments such as laser ablation of the layer, overall exposure of the element to actinic light, and washout to remove unexposed areas. Also the image bearing element does not form a relief structure suitable for flexographic printing.